An example of a process control unit of the general class discussed above is described in U.S. Pat. No. 4,596,144 (Stanley Panton and Steven J. Woodward) entitled "Acoustic Ranging System", the specification of which is incorporated herein by reference. Reference to FIG. 1 of the drawings of that patent will show that it incorporates a computer 28 provided with various connections to transducers, indicators and displays, and receiving inputs from a keyboard 52 and control keys 58.
U.S. Pat. No.4,596,144 describes methods of detecting a true echo in an ultrasonic range finding system which are essentially of a statistical nature, and not only identify an echo resulting from a particular shot, but are capable of quantifying the degree of assurance that a selected echo is a true echo. This latter information may be utilized in determining whether additional shots are required to provide reliable data.
All of the echo extraction techniques described in U.S. Pat. No. 4,596,144 have the following steps in common:
1. An echo profile is formed by taking one or more shots.
2. The first part of the echo profile is blanked in order to cover over the transmit pulse and some transducer ringing. Although the start of the echo profile coincides with the start of the transmit pulse, the useful information occurs after the end of blanking.
3. A reference curve is formed. The curve starts at a fixed start point and then follows the echo profile.
4. The most probably correct echo is selected by comparing the echo profile with the reference curve.
The problem arises in step 3. On the one hand it is desirable to set the start point low in order to confidently detect valid close in echoes. On the other hand it is desirable to set the start point high so that the reference curve will clear the unblanked portion of the transducer ringing, otherwise the ringing may be deemed to be the correct echo in step 4.
In the apparatus described in U.S. Pat. No. 4,596,144, the start point may be set manually by entering a value from the keyboard, or automatically. To set the start point automatically, the operator must first ensure that the material level is well down from the transducer, and then by use of the keyboard instructs the computer to calculate a start point which will cause the reference curve to clear the transducer ringing. The start point cannot be set with a full bin because the valid close in echo would appear to be transducer ringing and the start point would need to be set high to clear this echo.
A further problem arises because of variations in transducer ringing. The ringing may increase for the following reasons:
1. An increase or decrease in temperature.
2. A change in the mounting of the transducer; for example, the mounting bolts of the transducer may be tightened.
3. Natural aging of the transducer.
4. Replacement of the transducer.
The operator must recognize these factors and set the start point high enough to clear the worst case expected ringing. If the start point is too high then valid close in echoes will not be detected. If the start point is set too low then the apparatus may initially operate correctly, but a change of season will probably cause an increase in ringing and the start point must then be increased. If a compromise cannot be achieved then the blanking must be increased so that less of the ringing is seen. The disadvantage to increasing the blanking is that levels in the top portion of the bin cannot be measured, and the useful height of the bin is thus reduced.
In the transmitter design a trade off is made in selecting the transmit pulse width. A narrow pulse width has the effect of shifting the ringing to the left, when viewed graphically, simply because the end of transmission occurs sooner. The position of the echo remains the same and therefore close in echoes will stand out more above the ringing. A wide transmit pulse has the effect of producing the largest possible return echo, even in the presence of air currents which tend to disperse the sound wave, as often happens with distant targets.
Much effort has been directed to improving transducer performance, but in the present state of the art it is not possible to consistently manufacture a transducer with low and stable ringing while still maintaining other desirable features such as high sound output and rugged construction.
Although reliable operation throughout the full height of a bin is important, operation in the top region of the bin is frequently considered to be critical. A failure to indicate the correct level in the top region could result in the bin being overfilled. A solution to the problems described above would be highly desirable.